1. Field of the Invention
The present invention is a process whereby separation of aromatic hydrocarbons from aromatic and non-aromatic hydrocarbon feeds by pervaporation through selective membranes is improved by control of the amount of oxygen present in the feed. Maintenance of the feed oxygen concentration below 50 wppm, preferably below about 30 wppm, more preferably below 10 wppm, most preferably about 1 wppm and less, permits flux maintenance over the course of the pervaporation process. Oxygen levels in the feed can be maintained in or reduced to the recited low concentration ranges by use of oxygen scavengers or inhibitors such as hindered phenols or hindered amines.
Maintaining feed oxygen content levels at a low level has been found to be effective in preventing loss of flux during the course of the pervaporative separation of aromatic hydrocarbons from aromatic and non-aromatic feed mixtures. These feed mixtures are typically cracked hydrocarbon feeds exemplified by light cat naphtha, intermediate cat naphtha, heavy cat naphtha, jet fuel, diesel and coker gas oil, feed stocks which range from 65.degree. to 1050.degree. F. in boiling point.
2. Description of the Related Art
The removal of aromatic hydrocarbons from feed streams containing mixtures of aromatic hydrocarbons and non-aromatic hydrocarbons using membranes is a desirable process which has been described in the patent literature.
U.S. Pat. No. 2,947,687 teaches the separation of hydrocarbons by type through a non-porous membrane using a membrane solvent to enhance the permeation rate. Membrane solvents include substituted hydrocarbons which are soluble in and have solvent power for the membrane. The hydrocarbon solvent is an organic compound containing one or more atoms of halogen, oxygen, sulfur or nitrogen. Thus, materials such as carbontetrachloride, alcohols, ketones, esters, ethers, carboxylic acids, mercaptans, sulfides (e.g., diethylsulfide etc.), nitropropane, nitrobenzene, acetonitrile, formamide, ethylene diamine, etc. may be employed in an amount ranging from 1 to 100% based on total solvent to hydrocarbon feed. The process may be operated at a pressure differential between the feed and permeate zone with the permeate being removed by vacuum. Alternately the permeate can be removed by a sweep stream such as steam, air, butane, etc.
The membrane is non-porous and includes natural or synthetic rubber, vinyl polymers, cellulose esters, cellulose ethers.
The process can use any hydrocarbon source as feed and the separation achieved is in the order: saturated hydrocarbons, &lt;unsaturated hydrocarbons, &lt;aromatics. Saturated hydrocarbons of approximately the same boiling range permeate in the order of increasing selectivity: branched chain, &lt;cyclic-chain, &lt;straight chain configuration, i.e., straight chain paraffins more readily permeate through the membrane.
U.S. Pat. No. 3,140,256 teaches a membrane separation process employing a membrane comprised of a cellulose derivative (e.g. cellulose ester or ether) modified by reaction with aldehydes, organic di isocyanate, organic monoisocyanate, organo-phosphorus chlorides and organo-sulfur chlorides. Hydrocarbon feeds can be separated into these components by type using the membrane, e.g. aromatics can be separated from unsaturated hydrocarbon (olefins or di olefins) and/or from paraffins, or branched chain aliphatic hydrocarbons can be separated from other aliphatic hydrocarbons which have a different number of branched chains. Aromatic hydrocarbons permeate more rapidly than do the saturated (i.e. paraffinic) hydrocarbons. In an example methyl cyclohexane permeated through the membrane more selectively than did iso octane.
U.S. Pat. No. 3,370,102 teaches the membrane separation of aromatics from saturates in a wide variety of feed mixtures including various petroleum fractions, naphthas, oils, and other hydrocarbon mixtures. Expressly recited in '102 is the separation of aromatics from kerosene. The process produces a permeate stream and a retentate stream and employs a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type i.e. aromatics, unsaturated, saturated by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons by type, i.e. aromatics and/or olefins from gasoline boiling range mixtures by the selective permeation of the aromatics through certain cellulose ester non-porous membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid. U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation.
Polyurea/urethane membranes and their use for the separation of aromatics from non-aromatics are the subject of U.S. Pat. No. 4,914,064. In that case the polyurea/urethane membrane is made from a polyurea/urethane polymer characterized by possessing a urea index of at least about 20% but less than 100%, an aromatic carbon content of at least about 15 mole percent, a functional group density of at least about 10 per 1000 grams of polymer, and a C.dbd.O/NH ratio of less than about 8.0. The polyurea/urethane multi-block copolymer is produced by reacting dihydroxy or polyhydroxy compounds, such as polyethers or polyesters having molecular weights in the range of about 500 to 5,000 with aliphatic, alkylaromatic or aromatic diisocyanates to produce a prepolymer which is then chain extended using diamines, polyamines or amino alcohols. The membranes are used to separate aromatics from non-aromatics under perstraction or pervaporation conditions.
Thin film composites can be prepared either from suspension deposition as taught in U.S. Pat. No. 4,861,628 or from solution deposition as taught in U.S. Pat. No. 4,837,054.
The use of polyurethane imide membranes for aromatics from non-aromatics separations is disclosed in U.S. Pat. No. 4,929,358. The polyurethane-imide membrane is made from a polyurethane-imide copolymer produced by end capping a polyol such as a dihydroxy or polyhydroxy compound (e.g. polyether or polyester) with a di or polyisocyanate to produce a prepolymer which is then chain extended by reaction of said prepolymer with a di or polyanhydride or with a di or polycarboxylic acid to produce a polyurethane/imide. The aromatic/non-aromatic separation using said membrane is preferably conducted under perstraction or pervaporation conditions.
A polyester imide copolymer membrane and its use for the separation of aromatics from non-aromatics is the subject of U.S. Pat. No. 4,946,594. In that case the polyester imide is prepared by reacting polyester diol or polyol with a dianhydride to produce a prepolymer which is then chain extended preferably with a diisocyanate to produce the polyester imide.
U.S. Pat. No. 4,929,357 is directed to non-porous isocyanurate crosslinked polyurethane membranes. The membrane can be in the form of a symmetric dense film membrane. Alternatively, a thin, dense layer of isocyanurate crosslinked polyurethane can be deposited on a porous backing layer to produce a thin film composite membrane. The isocyanurate crosslinked polyurethane membrane can be used to separate aromatic hydrocarbons from feed streams containing mixtures of aromatic hydrocarbons and non-aromatic hydrocarbons, the separation process being conducted under reverse osmosis, dialysis, perstraction or pervaporation conditions, preferably under perstraction or pervaporation conditions.
U.S. Ser. No. 452,887, filed Dec. 19, 1989 in the names of Black and Schucker, now U.S. Pat. No. 4,962,271 teaches the selective separation of multi-ring aromatic hydrocarbons from distillates by perstraction. The multi-ring aromatics are characterized by having less than 75 mole % aromatic carbon content. Perstractive separation is through any selective membrane, preferably the aforesaid polyurea/urethane, polyurethane imides or polyurethane isocyanurates.